Recently, environmental problems on global scale are gathering keen attention. In also the automotive industry, attempts for environmental improvement in the manufacturing steps are positively advanced. Manufacturing steps of automotives give rise to such problems as global warming, generation of industrial waste and discharge of volatile organic compounds (VOC). In particular, most of the VOC is discharged during the coating steps and effective countermeasure is of urgent necessity.
Outer panel portion of automobile bodies are normally coated with multilayer coating film formed of undercoat film of cationic electrocoating, intermediate coat film and top coat film, for imparting corrosion resistance and for aesthetic purpose. With the view to reduce VOC discharge, use of water-based paints for intermediate and top coatings is promoted.
Coating film on automobiles is, moreover, required to excel in the film performance, in particular, high finished appearance, chipping resistance (resistance to damages by gravel knocked off by running cars) and water resistance. In respect of the chipping resistance, the intermediate coating film plays particularly important role.
However, conventional water-based paint in general is inferior in coating film performance such as mechanical properties and water resistance, as compared with organic solvent-based paint.
As water-based intermediate paint, generally water-based paint compositions whose chief components are polyester resin and hardening agent are used. Where aliphatic polybasic acid-derived polyester resin is used as the polyester resin in such compositions, the resulting coating film excels in chipping resistance but storage stability of the paint tends to become inferior.
Whereas, when polyester resin formed with use of aromatic polybasic acid is used, the paint shows insufficient storage stability and its coating film tends to have inferior chipping resistance. Thus in developing water-based intermediate paint, optimization of the polyester resin is in demand.
To meet such demand, JP 2004-67995A discloses that aqueous resin compositions comprising polyester resin, whose at least 70% of polybasic acid component is aromatic acid and alicyclic acid, specific vinyl-modified polyester resin and hardening agent, exhibit excellent storage stability and can form coating film excelling in chipping resistance. However, coating films obtained of the aqueous resin compositions occasionally show inferior smoothness and water resistance.
JP 2004-292775A discloses that resin compositions for water-based paint which comprise polyester resin, whose at least 70 mol % of polybasic acid component is aromatic acid and alicyclic acid, vinyl-modified epoxy ester resin and hardening agent, exhibit excellent storage stability and can form coating film excelling in chipping resistance. However, coating films obtained of the aqueous resin compositions occasionally show inferior smoothness and water resistance.
JP 2004-2729A discloses that a water-based intermediate paint comprising polyester resin whose essential constituent components are alicyclic polybasic acid and/or alicyclic polyhydric alcohol, other polybasic acid and other polyhydric alcohol; crosslinking agent; and aqueous urethane resin emulsion; shows favorable stability and can form multilayer film excelling in chipping resistance and finished appearance. However, coating films obtainable from the water-based intermediate paint occasionally fail to have sufficient distinctness of image.
JP Hei 4 (1992)-93374A discloses that a water-based intermediate paint whose chief components are polyester resin having specific acid value, hydroxyl value and number-average molecular weight, aqueous amino resin, specific low molecular weight polyesterdiol and alkyl-etherified benzoin possesses excellent coating workability and can form coating film of excellent chipping resistance, smoothness and moistureproofness. The coating films obtained of the water-based intermediate paint, however, are occasionally inferior in water resistance.
Also International Publication WO 04/61025 discloses a paint composition which contains a copolymer resin emulsion to which self crosslinkability is imparted and which has a Tg, acid value and hydroxyl value each falling within a specific range, and a multilayer coating film-forming method using the paint composition, as a water-based intermediate paint composition which, when used to form a multilayer coating film, gives high chipping resistance and water resistance, and shows good compatibility with top coat and undercoat, and also excellent finished appearance, and a multilayer coating film-forming method using the composition. The weight-average molecular weight of the copolymer resin emulsion used therein is, however, around 50,000-1,000,000 and its effect for improving coating film performance such as chipping resistance is insufficient.
While it is generally effective for improving coating film performance to use high molecular weight resins (in particular, acrylic resin), there is a problem that use of, for example, intraparticulate crosslinking type high molecular weight acrylic resin emulsion degrades finished appearance such as coated surface smoothness, and coating workability.
JP 2004-137504A discloses aqueous acrylamide polymer solution having a weight-average molecular weight of 500,000-10,000,000 at specific concentration and viscosity ranges, as an aqueous high molecular weight polymer solution. The main utility of the polymer, however, is paper reinforcing agent. It is unsuitable for use in paint, because it shows unsatisfactory finished appearance and coating workability as a high molecular weight acrylic resin for paint use and coating film formed thereof has insufficient water resistance.